Plane-wave Response Research Articles

Modeling sea surface noise in the presence of seamounts and ocean fronts using Nx2D and 3-D methods

This study focuses on modeling the ambient noise observed along a vertical line array in the presence of distributed surface sources and extreme bathymetric and oceanographic features. The distributed surface sources considered in this study include wind-induced noise, while the extreme bathymetric and oceanographic features comprise seamounts such as Atlantis II and the ocean front caused by the Gulf Stream. Directional vertical noise levels are modeled by computing the Green’s Function between the array elements and the distributed surface sources, estimating the average cross spectral density matrix, and calculating the plane wave response [1]. Green’s functions are computed using the Bellhop3-D ray tracing model [2] in both an Nx2D and full 3-D environment, and the resulting directional noise levels are compared. The results of this study will provide insights into the accuracy and effectiveness of the modeling methods used to predict ambient noise levels in such challenging environments, with particular emphasis on the contribution of seamounts to ambient noise. The implications of these results can be significant for thedesign and operation of underwater acoustic systems in similar environments.

Design of X-band bicontrollable metasurface absorber comprising graphene pixels on copper-backed YIG substrate

The plane wave response of a bicontrollable metasurface absorber with graphene-patched pixels was simulated in the X band using commercial software. Each square meta-atom is a 4 × 4 array of 16 pixels, some patched with graphene and the others unpatched. The pixels are arranged on a PVC skin which is placed on a copper-backed YIG substrate. Graphene provides electrostatic controllability and YIG provides magnetostatic controllability. Our design delivers absorptance ≥0.9 over a 100-MHz spectral regime in the X band, with 360 MHz kA−1 m magnetostatic controllability rate and 1 MHz V−1μm electrostatic controllability rate. Notably, electrostatic control via graphene in the GHz range is novel.

Data-driven retrieval of primary plane-wave responses.

ABSTRACTSeismic images provided by reverse time migration can be contaminated by artefacts associated with the migration of multiples. Multiples can corrupt seismic images, producing both false positives, that is by focusing energy at unphysical interfaces, and false negatives, that is by destructively interfering with primaries. Multiple prediction/primary synthesis methods are usually designed to operate on point source gathers and can therefore be computationally demanding when large problems are considered. A computationally attractive scheme that operates on plane‐wave datasets is derived by adapting a data‐driven point source gathers method, based on convolutions and cross‐correlations of the reflection response with itself, to include plane‐wave concepts. As a result, the presented algorithm allows fully data‐driven synthesis of primary reflections associated with plane‐wave source responses. Once primary plane‐wave responses are estimated, they are used for multiple‐free imaging via plane‐wave reverse time migration. Numerical tests of increasing complexity demonstrate the potential of the proposed algorithm to produce multiple‐free images from only a small number of plane‐wave datasets.

Appropriate Nonlocal Hydrodynamic Models for the Characterization of Deep‐Nanometer Scale Plasmonic Scatterers

AbstractThe interaction between light and plasmonic systems with deep‐nanometer characteristics, which is essentially governed by quantum mechanical effects, has been extensively studied by the nonlocal hydrodynamic approach. Several hydrodynamic models, supplemented by additional boundary conditions, have been introduced in order to describe the collective motion of the free electron gas in metals. Four hydrodynamic models, namely the hard wall hydrodynamic model (HW‐HDM), the curl‐free hydrodynamic model, the shear forces hydrodynamic model, and the quantum hydrodynamic model (Q‐HDM), are thoroughly investigated. The investigation studies the mode structure (the natural modes or, in quantum optics, the quasi‐normal modes) of the spherical core–shell topology, which is complemented by the plane wave response from the system. The results of the above hydrodynamic models are also compared with those of the specular reflection method. It is demonstrated that the choice of a particular hydrodynamic model strongly affects the natural frequencies and modes in the mode structure of the topology and thus drastically modifies the simulated fields in the near and far regions. Contrary to HW‐HDM and Q‐HDM, the other two hydrodynamic models fail to predict the particles’ response accurately, showing artifactual mode hybridization.

Planewave response of a simple Lorentz-nonreciprocal medium with magnetoelectric gyrotropy

The simple Lorentz-nonreciprocal medium described by the constitutive relations D = ε0εrE − Γ × H and B = μ0μrH − Γ × E is inspired by a specific spacetime metric, Γ being the magnetoelectric-gyrotopy vector. Field representations in this medium can be obtained from those for the isotropic dielectric-magnetic medium. When a plane wave is incident on a half space occupied by the Lorentz-nonreciprocal medium with magnetoelectric gyrotopy, theory shows that the transverse component of the magnetoelectric-gyrotopy vector is responsible for a rotation about the normal axis; furthermore, left/right reflection asymmetry is exhibited. Additionally, left/right transmission asymmetry is exhibited by a planar slab composed of the Lorentz-nonreciprocal medium with magnetoelectric gyrotopy. The left/right asymmetries are of interest for one-way devices.

Three Dimensional Image-Based Radar Cross Section Extrapolation via Planar Projective Transforms

Conventional radar cross section (RCS) measurements require far-field or compact-antenna-test-range (CATR) conditions, which have strict restrictions and high implementation costs. By contrast, RCS extrapolation methods in the near zone are more convenient and practical, which become the research emphasis in recent years. This paper presents a novel RCS extrapolation method based on the combination of near-field 3D synthetic aperture radar (SAR) technique and planar projective transforms (PPT) algorithm. Firstly, to extract the target’s reflectivity distribution, we apply near-field 3D SAR imaging and obtain 3D near-field RCS (NFRCS) images. Secondly, to meet the requirement of RCS measurements, we derive a novel correction factor used to precisely expand 3D NFRCS images. Finally, we extrapolate the plane-wave responses in the azimuth-vertical dimensions, which presents as planar patterns. The proposed method utilizes the complete 3D image-based information to overcome the inherent constraints of classical RCS extrapolation methods on application scenarios, which has the advantages of broad applicability and high flexibility. The detailed derivation and implementation of this method are described in this paper. The simulation and experiment results verify that the proposed method can provide the equivalent CATR result within ±11° azimuth and elevation angles, and it is applicable to precisely measuring the point, surface, and complex scatterers.

Incoherent Detection of Orthogonal Polarizations via an Antenna Coupled MKID: Experimental Validation at 1.55 THz

There is an increasing demand for large format detector arrays with large bandwidths and high antenna efficiencies for future THz astronomical radiometric applications. For direct detection instruments, it is also desired to have antennas with dual polarization reception in order to increase the received power from incoherent sources, thereby improving the observing speed of the instrument. The main goal of this work is the validation of the incoherent detection of two orthogonal polarizations by a leaky lens antenna, coupled to a single microwave kinetic inductance detector (MKID). The resonant frequency of the MKID changes depending on the absorbed power over a distributed transmission line. The proposed antenna is composed of two crossed leaky wave slots feeding a silicon extended hemispherical lens. The slots are coupled to four aluminum (Al) coplanar waveguide (CPW) lines that incoherently absorb the incoming THz radiation. The antenna and the power absorbing CPW lines are embedded inside the MKID, allowing efficient radiation detection at THz frequencies where no lossless superconductors are available. The proposed dual-polarized device absorbs power incrementally over four different CPWs incoherently and is therefore simulated in reception (deriving a plane-wave response) similarly to what is done in distributed absorbers. We compare numerically and experimentally the proposed dual-polarized leaky lens coupled MKID and its single polarization counterpart and show that the dual polarized device receives twice as much power as the single-polarized one. Eventually, the dual-polarized device, when used with air-bridges, provides the same angular selectivity and twice the throughput of the single-polarized one.

Virtual plane-wave imaging via Marchenko redatuming

Marchenko redatuming is a novel scheme used to retrieve up- and down-going Green's functions in an unknown medium. Marchenko equations are based on reciprocity theorems and are derived on the assumption of the existence of so called focusing functions, i.e. functions which exhibit time-space focusing properties once injected in the subsurface. In contrast to interferometry but similarly to standard migration methods, Marchenko redatuming only requires an estimate of the direct wave from the virtual source (or to the virtual receiver), illumination from only one side of the medium, and no physical sources (or receivers) inside the medium. In this contribution we consider a different time-focusing condition within the frame of Marchenko redatuming and show how this can lead to the retrieval of virtual plane-wave responses, thus allowing multiple-free imaging using only a 1 dimensional sampling of the targeted model. The potential of the new method is demonstrated on a 2D synthetic model.

Holonomy, quantum mechanics and the signal-tuned Gabor approach to the striate cortex

It has been suggested that an appeal to holographic and quantum properties will be ultimately required for the understanding of higher brain functions. On the other hand, successful quantum-like approaches to cognitive and behavioral processes bear witness to the usefulness of quantum prescriptions as applied to the analysis of complex non-quantum systems. Here, we show that the signal-tuned Gabor approach for modeling cortical neurons, although not based on quantum assumptions, also admits a quantum-like interpretation. Recently, the equation of motion for the signal-tuned complex cell response has been derived and proven equivalent to the Schrödinger equation for a dissipative quantum system whose solutions come under two guises: as plane-wave and Airy-packet responses. By interpreting the squared magnitude of the plane-wave solution as a probability density, in accordance with the quantum mechanics prescription, we arrive at a Poisson spiking probability — a common model of neuronal response — while spike propagation can be described by the Airy-packet solution. The signal-tuned approach is also proven consistent with holonomic brain theories, as it is based on Gabor functions which provide a holographic representation of the cell’s input, in the sense that any restricted subset of these functions still allows stimulus reconstruction.

The pulsed EM plane-wave response of a thin planar antenna

Electromagnetic coupling of an impulsive plane wave to a thin planar antenna is described analytically using the reciprocity theorem of the time-convolution type. It is shown that the pulsed voltage response induced within a thin planar antenna can be expressed through a simple rim integration of either the “testing” voltage distribution or the generalized-ray representation of the time-domain Green’s function. Numerical experiments demonstrate the validity and high computational efficiency of the proposed closed-form coupling model.

Plane-Wave Synthesis and RCS Extraction via 3-D Linear Array SAR

This letter presents an improved back-projection (BP) technique for far-field radar cross section (RCS) feature extraction from near-field measurement. The presented technique can synthesize a plane-wave response from a 2-D near-field planar scanner and obtain a 3-D scattering coefficient image. Due to the 3-D resolving power, it can spatially extract different parts of a complex object, or target’s scattering coefficient from the whole image which may include environment noise such as ground and support. From the 3-D scattering coefficient image, far-field RCS can be obtained as a function of direction and frequency. Simulations of RCS feature extraction for three squares and a 3-D complex-shaped electric-large car model are presented to validate the improved BP method.

Determination of isotropic layer parameters from spatiotemporal signals of an ultrasonic array

The paper discusses a method for measuring the velocities and attenuation of longitudinal and transverse ultrasonic waves and the density and thickness of the isotropic layer with an array placed in an immersion liquid parallel to the sample. The method is based on the recording of the total spatiotemporal signal of the array and its expansion into a spatial spectrum of pulse plane wave response. The ultrasonic velocity and sample thickness depend on the response delay of the plane wave in the layer from the transverse projection of the slowness vector. The density and attenuation are determined from the behavior of the amplitudes of spectral responses. To confirm this method in experiment, the parameters of a polystyrene plate have been measured using a linear 32-element array with a central frequency of 17 MHz.

Modelling ultrasonic array signals in multilayer anisotropic materials using the angular spectrum decomposition of plane wave responses

Ultrasonic arrays have seen increasing use for the characterisation of composite materials. In this paper, ultrasonic wave propagation in multilayer anisotropic materials has been modelled using plane wave and angular spectrum decomposition techniques. Different matrix techniques, such as the stiffness matrix method and the transfer matrix method, are used to calculate the reflection and transmission coefficients of ultrasonic plane waves in the considered media. Then, an angular decomposition technique is used to derive the bounded beams from finite-width ultrasonic array elements from the plane wave responses calculated earlier. This model is considered to be an analytical exact solution for the problem; hence the diffraction of waves in such composite materials can be calculated for different incident angles for a very wide range of frequencies. This model is validated against experimental measurements using the Full-Matrix Capture (FMC) of array data in both a homogeneous isotropic material, i.e. aluminium, and an inhomogeneous multilayer anisotropic material, i.e. a carbon fibre reinforced composite.

Characterization of directional microphones in an arbitrary sound field

An acoustic characterization method for directional microphones is presented that does not require an anechoic chamber to provide a controlled plane wave sound field. Measurements of a directional microphone under test are performed in a nearly arbitrary sound field for several angles of sound incidence, and the corresponding sound pressure and pressure gradients in the vicinity of the test microphone are measured using an automated probe microphone scanning system. From these measurements, the total acoustic frequency response of the directional microphone can be decomposed into its sensitivities to sound pressure and pressure gradient using a least squares estimation technique. These component responses can then be combined to predict the directional response of the microphone to a plane wave sound field. This technique is demonstrated on a commercially available pressure gradient microphone, and also on a combination sound pressure-pressure gradient microphone. Comparisons with the plane wave responses measured in an anechoic environment show that the method gives accurate results down to 100 Hz.

The Influence of Spatial Filters on Infrasound Array Responses

A spatial filter is often attached to a microphone or microbarometer in order to reduce the noise caused by atmospheric turbulence. This filtering technique is based on the assumption that the coherence length of turbulence is smaller than the spatial extent of the filter, and so contributions from turbulence recorded at widely separated ports will tend to cancel while those of the signal of interest, which will have coherence length larger than the spatial dimensions of the filter, will be reinforced. In this paper, the plane wave response for a spatial filter with an arbitrary arrangement of open ports is determined. It is found that propagation over different port-to-sensor distances causes out-of-phase sinusoids to be summed at the central manifold and can lead to significant amplitude decay and phase delays as a function of frequency. The determined spatial filter plane wave response is superimposed on an array response typical of infrasound arrays that constitute the International Monitoring System infrasound network used for nuclear monitoring purposes. It is found that signal detection capability in terms of the Fisher Statistic can be significantly degraded at certain frequencies. The least-squares estimate of signal slowness can change by up to 1.5° and up to 10 m/s if an asymmetric arrangement of low and high frequency spatial filters is used. However, if a symmetric arrangement of filters is used the least-squares estimate of signal slowness is found to be largely unaffected, except near the predicted null frequency.

FDTD3C—A FORTRAN program to model multi-component seismic waves for vertically heterogeneous attenuative media

Full-waveform seismic response of horizontally layered media can be calculated by semi-analytical methods. However, for gradient velocity and randomly heterogeneous structures the semi-analytical methods face difficulties. In such cases, numerical methods such as the finite-difference (FD) method have to be used. We develop an efficient numerical scheme to calculate plane-wave response of vertically heterogeneous attenuative media by applying Radon transform to the three-dimensional wave equation. The scheme employs fourth-order FD operator in space and second-order FD operator in time to solve the wave equation. In order to facilitate applicability of the scheme we introduce the FORTRAN code FDTD3C which implements the algorithm and provides multi-component response of the media to oblique incident P-, SV-, and SH-waves incoming from arbitrary azimuth. The calculated components are three particle velocity components in three Cartesian directions, and divergence and rotation of the wavefield. The code is extremely efficient and is capable of incorporating highly fluctuating subsurface velocity and attenuation models. This program is intended for all FD users who are concerned with full-waveform seismic modelling and inversion. Wide range of applicability of the code is demonstrated with a set of numerical examples.

Validation of microwave radiometry for measuring the internal temperature profile of human tissue

A phantom target with a known linear temperature gradient has been developed for validating microwave radiometry for measuring internal temperature profiles within human tissue. The purpose of the phantom target is to simulate the temperature gradient found within the surface layers of a baby's brain during hypothermal neuroprotection therapy, in which the outer surface of the phantom represents the skin surface and the inner surface the brain core. The target comprises a volume of phantom tissue material with similar dielectric properties to high water-content human tissue, contained between two copper plates at known temperatures. The antenna of a microwave radiometer is in contact with one surface of the phantom material. We have measured the microwave temperature of the phantom with microwave radiometry in a frequency band of 3.0–3.5 GHz. Our microwave temperature measurements have small 0.05 °C (type A) uncertainties associated with random effects and provide temperatures consistent with values determined using theoretical models of the antenna–target system within uncertainties. The measurements are in good agreement with the major signal contribution being formed over a near plane-wave response within the material with a much smaller contribution from close to the antenna face.

Three-dimensional, wavefield imaging of broadband seismic array data

This paper describes an implementation of a three-component, three-dimensional plane wave migration method to image P-to-S converted waves that arrive following the direct P waves of teleseismic earthquakes. The programs described assume that the input data have been deconvolved to produce a set of vector impulse response functions with the initial P pulse centered at time 0. This method was implemented as three different application programs called pwstack, pwmig, and gridstacker. pwstack produces local-scale plane wave decompositions using the pseudostation concept. The output of pwstack is an estimate of the plane wave response over a grid of slowness vectors interpolated onto a regular grid in space. The Gaussian smoother that is used for the pseudostation method has the additional benefit of serving as a type of spatial antialiasing filter. The program called pwmig takes the output of pwstack and inverts the plane wave data for radial and transverse scattering potential using an inverse generalized Radon transform. Data can be back-projected using a conventional longitudinal, radial, transverse coordinate system or a novel depth-variable method that computes a transformation matrix linking each vector sample to an equivalent dipping-layer coordinate system at the corresponding scattering point. Because pwmig is a prestack method, a final program, gridstacker, is needed to stack data from multiple events that characterize real data. gridstacker is a general solution for stacking data with different grid geometries and variable data quality. Weighting functions can be defined externally by a user-supplied recipe or one can use a set of robust estimation methods coded in gridstacker. The programs are validated with simulation data produced by recently developed elastic finite-difference code and a model that simulates the edge of a subduction zone. Finally, I show an example of an application of these programs to image the western United States with the USArray.

Modeling columnar thin films as platforms for surface–plasmonic–polaritonic optical sensing

Via exploitation of surface plasmon polaritons (SPPs), columnar thin films (CTFs) are attractive potential platforms for optical sensing as their relative permittivity dyadic and porosity can be tailored to order. Nanoscale model parameters of a CTF were determined from its measured relative permittivity dyadic, after inverting the Bruggeman homogenization formalism. These model parameters were then used to determine the relative permittivity dyadic of a fluid-infiltrated CTF. Two boundary-value problems were next solved: the first relating to SPP-wave propagation guided by the planar interface of a semi-infinitely thick metal and a semi-infinitely thick CTF, and the second to the plane-wave response of the planar interface of a finitely thick metallic layer and a CTF in a modified Kretschmann configuration. Numerical studies revealed that SPP waves propagate at a lower phase speed and with a shorter propagation length, if the fluid has a larger refractive index. Furthermore, the angle of incidence required to excite an SPP wave in a modified Kretschmann configuration increases as the refractive index of the fluid increases.

Negatively refracting chiral metamaterials: a review

Chirality extends the class of negatively refracting metamaterials by endowing a richer palette of electromagnetic properties. Chiral metamaterials can support negative refraction, which must be assessed in light of the closely related phenomenons of negative phase velocity and counterposition. Two categories of chiral metamaterials are being examined these days: (a) homogeneous and homogenizable chiral materials, as exemplified by isotropic chiral materials, Faraday chiral materials, and materials with simultaneous mirror-conjugated and racemic chirality characteristics; and (b) structurally chiral materials, as exemplified by helicoidal bianisotropic materials and ambichiral materials. The planewave response of a half-space occupied by a chiral metamaterial is complex, and important distinctions between negative refraction, negative phase velocity, and counterposition emerge.